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How to keep only one program running

How can I keep only one windows program running in memory?
What should I check when this program starts running?
1 Solution
Tommy HuiEngineerCommented:
Do you want only one Windows program running? Regardless of what that program is?

Which version of Windows are you referring to? 16-bit or 32-bits?
If you mean one instance of a given program, there are several ways of accomplishing this.

In Win16, the preferred way is to check the previous instance parameter to WinMain().

In Win32 you cannot do that so you can create a global object (like semaphore or event) with a hardcoded name and check the return value to see if the object already existed.

hbhAuthor Commented:
Yes, I just want one window program running under windows 95,
a 32bit platform. An example called Spy++ in MSVisual C++ is
such as this kind of program, which has only one copy when it
is running.

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Here's how I'd do it for Win32:

#pragma data_seg("myapplication")
BOOL g_bHasStarted;

Then, in the initialization somewhere:

if(g_bHasStarted == 0) {
   g_bHasStarted = 1;
} else {
   Exit();   /* or your cleanup routine. */

that works too but only if you instruct the linker to make the "myapplication" segment shared, *and* initialize the global variable.

Something like:

#pragma comment(linker, "/section:myapplication,rws")
#pragma data_seg("myapplication")
BOOL g_bHasStarted = 0;
#pragma data_seg()

However, it relies on #pragmas specific to the MSVC compiler and thus is less portable.

I'd suggest relying on a named object or a global atom.

Something like:

    hMutex = CreateMutex (NULL, FALSE, "Some unique name");
    if ((hMutex != NULL) && (GetLastError () == ERROR_ALREADY_EXISTS)) {
        MessageBox (NULL, "Another instance of this application is running",
            "Error", MB_OK | MB_ICONSTOP);
        CloseHandle (hMutex);
        return -1;

In the beginning of your program, and:

    CloseHandle (hMutex);

At the end.

hbhAuthor Commented:
I have adopted alexo's method. It is simple and effictive.
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